p21(c-Ha-Ras) (Ras) can be activated by the guanine nucleotide exchange factor mSOS1 or by S-nitrosylation of cysteine 118 via nitric oxide (NO). To determine whether these two Ras-activating mechanisms modulate distinct biological effects, a NO-nonresponsive Ras mutant (Ras(C118S)) was stably expressed in the PC12 cells, a cell line that generates NO upon nerve growth factor treatment. We report here that Ras(C118S) functions indistinguishably from wild type Ras in activating and maintaining the mSOS1- and Raf-1-dependent mitogen-activated protein kinase cascade necessary for neuronal differentiation. However, continuous (>5 days) exposure to nerve growth factor reveals that, in contrast to parental or wild-type Ras-overexpressing PC12 cells, Ras(C118S)-expressing PC12 cells cannot sustain the basal interaction between Ras and phosphatidylinositol 3-kinase. This results in spontaneous apoptosis of these cells despite the presence of nerve growth factor and serum. Thus unique downstream effector interactions and biological outcomes can be differentially modulated by distinct modes of Ras activation.